The present invention relates to nickel electro-plating. It relates to the field of electro-plating of metal or non-metal parts with nickel by means of plating baths using nickel sulfamate as the nickel providing species.
An important application lies in internally nickel electro-plating hollow elements such as the tubes of a steam-generator, pressurizer nozzles, or adapters for the reactor vessel lid in a pressurized water nuclear power station, or any other tube used in the nuclear industry or elsewhere. Another important application lies in nickel plating electrical connector elements. However, it relates more generally to any nickel plating that makes use of a nickel sulfamate bath either by means of a static process (the bath being in a vessel having fixed electrodes), or by means of a dynamic process (the bath being caused to flow or the parts to be nickel-plated being moved).
The invention further relates to purifying nickel plating baths used for nickel plating.
It is known that nickel plating the inside zones in U-shaped tubes of a steam generator that are subjected to particularly severe stresses makes it possible to close microcracks or to prevent such cracking. As an example of a known nickel plating repair method, reference may be made to EP-A-0 167 513.
The apparatuses conventionally used for nickel electro-plating comprise:
a receptacle containing the nickel plating bath, an anode often constituted by a screened basked (e.g., made of titanium) and filled with balls of sacrificial nickel (e.g., those sold by the firm INCO), and a cathode constituted by the part to be nickel plated; or PA1 the same or similar receptacle, nickel plating bath, and anode as above, but with a cathode constituted by a metal plate (e.g., stainless steel), with the entire apparatus then being used for preparing the nickel plating bath prior to use by purifying it to eliminate undesired metal impurities by controlled electrolysis (e.g., eliminating cobalt for nuclear installations); or PA1 a hollow cylindrical anode coated with sacrificial nickel and placed in the center of the tube to be repaired by nickel plating, which tube constitutes the cathode, the nickel plating bath following in one direction between the cathode and the anode and then the opposite direction inside the anode.
With the devices described above, complex compounds are formed during electrolysis, both on the cathode and on the anode, and can recombine freely with each other.
The term "complex compounds" is used to designate compounds that drive from a change in the sulfamate bond, such as azodisulfonate.
These complex compounds present problems during nickel plating which are generally manifested by the nickel anode being passivated, in particular because of the reduction on the cathode of oxygen compounds formed at the anode which unbalance the electrochemical system towards a potential where the anode becomes passivated, and/or by increase in the electromechanical resistance of the nickel plating cell due, in particular, to the presence of poorly ionized compounds, which leads to operation which is no longer under the conditions of current and voltage that are desirable for nickel plating, to coatings which no longer have the required quality, and/or rapid degradation of the nickel plating baths.
The abstract "Nickel electro-plating cell", Hitachi Ltd., Japan Kokai Tokyo Koho JP 59 193 295, page 500, col. 2, Chemical Abstracts, Vol. 102, No. 18, May 1985, describes a cell having a cation exchanging membrane permeable to Ni-ions for separating a Pt-coated anode and a cathode. There is no reference to a barrier other than a ion-exchanging membrane.